Process and apparatus for drying and treating lumber



NOV- 8, 1966 P. r. FARNSWORTH 3,283,412

PROCESS AND APPARATUS FOR DRYING AND TREATING LUMBER INVENTOR P/r/Za Z'Farwswarz,

WMS/mm Nov.y 8, 1966 P. T. FARNswoRn-l PROCESS AND PPARATUS FOR DRYING AND TREATING LUMBER Filed Sept. 9, 1964 2 Sheets-Sheet 2 I NVENTOR.

MS r W Sura. wwf r o m Kw4 /m w United States Patent O PROCESS AND APPARATUS FOR DRYING AND TREATING LUlVIBER Philo T. Farnsworth, Fort Wayne, Ind., `assignor of onehalf interest to Frederick R. Furth, Cornwall Bridge, Conn.

Filed Sept. 9, 1964, Ser. No. 395,250 Claims. (Cl. 34-5) The present invention relates to a method and apparatus for the rapid removal of moisture from green lumber and the like. It also relates to a method for the coloring and/ or impregnating of dried lumber.

In the most common treatment of green lumber, the boards are roughly cut to predetermined sizes and then either air or kiln dried. Air or natural drying requires from one to as many as five years, depending upon the woods density and moisture content. Kiln drying can reduce the time required to as little as a few weeks. During these processes, however, all boards change their dimensions (shrink) and some will warp and/or develop checks. Badly checked boards must be scrapped; the same is true for warped boards, unless there is suiiicient material to permit planing out the distortions. All of these add up to economic losses to the lumber industry.

A number of systems have been proposed or tried for more rapid removal of moisture from green lumber. These processes generally have employed radio-frequency heating or chemical solvents. They have been found unsatisfactory for one reason or another. In the radiofrequency drying process, the control becomes extremely complex,I because the resistivity of the lumber varies with its moisture content and thus requires almost continuous compensating adjustments to the applied energy source to prevent scorching the boards. Obviously, this process cannot be used with a mixture of woods of differing moisture contents. Chemical methods have also been proposed but these methods are much slower in the removal of moisture than by radio-frequency heating, and tend to produce alterations in the characteristics of the woods by changes in the natural colors thereof, loosening of knots, weakening and straining of the cellulose fibers and the like. As in air and kiln drying, the lumber changes dimensions and is subject to warping and checking.

It is therefore an object of this invention to provide an improved apparatus and method for the drying of lumber in the respects of greatly increasing the speed of drying and providing more uniformity and controllability during drying.

It is another object of this invention to provide an apparatus and method for 'drying lumber wherein alteration of characteristics of the lumber as a consequence of the drying are minimized and problems involved in the weakening or distortion of fibers, loosening of knots, and alteration of color are substantially eliminated.

It is another object of this invention to provide an apparatus and method for not only drying lumber but also for coloring and impregnating the lumber to any degree desired during the drying process.

It'is another object of this invention to provide an apparatus and method for drying lumber wherein no changes in dimensions or raising of the wood grain takes place during the drying, and green boards can be cut and planed to nished dimensions prior to drying.

It is still another object of this invention to provide an apparatus for drying lumber which is reliable in operation, efficient in use and is easily maintained.

The above-mentioned and other features and objects of this invention and the manner of attaining .them will become more apparent and the invention itself will be 3,283,412 Patented Nov. 8, 1966 best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. l is a diagrammatic illustration, in side elevation, of one embodiment of this invention, in solid lines, and of a modification of this embodiment in dotted lines;

FIG. 2 is a similar illustration, in section, taken substantially along section lines 2-2 of FIG. l;

FIG. 3 is a fragmentary view, partially in cross-section, of the apparatus of FIG. l, taken substantially along the section line 3-3 of FIG. 2;

FIG. 4 is a front elevational view, partially broken away for clarity of illustration and partially sectioned, of the chamber shown in FIG. 1 in a slightly different form;

FIG. 5 is an enlarged, perspective fragmentary view of an alternative arrangement for sticking the lumber and subliming the ice; and

FIG. 6 is a cross-sectional view of the stick of FIG. 5 with sections of lumber being shown in position with respect thereto.

Referring to the drawings, and more particularly to FIGS. l through 3, the apparatus there illustrated cornprises a relatively large steel enclosure or chamber 10 which is of such construction and material that it is capable of withstanding the forces and conditions to which it is subjected as will be explained more fully later on. In a practical working embodiment of this invention, the ends 12, 14 are provided with hermetically sealed doors 58 which may be hinged for opening the chamber completely. Instead of using two-hinged doors 58 as shown in FIG. 4, an oval-shaped closure may be used which seals against a suitable gasket at the end of the chamber 10 when the chamber is being closed. In shape, the chamber 10 has an elliptical or oval cross-section and a length longer than the major axis of the aforementioned cross-section. In one embodiment of this invention, its minor axis measures approximately twenty (20) feet and its length about forty (40) feet. Other sizes may be used without departing from the scope of this invention. v

In the exemplary embodiment shown, mounted on the inner walls of the opposite ends 12 and 14 of the chamber 10 are two relatively strong, horizontal ledges 16 and 18 which support the ends of two longitudinally extending, suitably strong beams 20. These beams may be constructed of either wood or steel and may be supported at several points along the lengths thereof by additional brackets (not shown) attached to the side walls of the chamber 10.

A high capacity vacuum pump 22 is connected to the chamber i0 at about midway between the ends as shown by means of a conduit 24 which, in the example previously given, is about twenty-four (24) inches in diameter. A refrigerating apparatus 26 in one embodiment of this invention (solid lines in FIG. l) is connected in series with a conduit 28 which empties into the left-hand, bottom portion of the chamber 10 as shown. A heat exchanger 33 may be substituted for the refrigerator 26 in another embodiment of this invention (dotted lin-es of FIG. l). A discharge line 30 leads from the righthand end of the chamber 10 as shown to a reclaiming or distilling apparatus indicated by the numeral 32. This apparatus 32 is provided with a drain 34 which serves a purpose to be described hereinafter. Connected in series between the refrigerator 26 or heat exchanger 33 and the reclaimer 32 is a pump 36 whereby liquid may be drawn from the right-hand end of the chamber 10 via the pipe 30 and forced through the refrigerator 26 or heat exchanger 33 and into the left-hand end of the chamber.

3 A source of dry heated gas at a super-atmospheric pressure and temperature is indicated by the numeral 38 l l and is `shown as being connected by means of a coupling 41 and a flexible line 40 'to the chamber 10.

This line 40 is shown as extending through the end wallr12 for a short distance so as to provide a nozzle portion 42. In a specific embodiment, the .dry gas may be heated i Within chamber 10 by radiation from -a steam coil with- 3 in chamber 10 which surrounds the volume of the boards 48 and S0.

The components thus far described, namely the vacuum fpump 22, the refrigerator 26, heat exchanger 33, the j reclaimer 32the pump 36 and the gas -source 38, are 4; individually conventional and will need no further specific 1 description than will be given in the following discussion.

Briefly, however, it may be stated that the vacuum pump j 22 preferably is of the stea-m jet, vacuum type which has adequate capacity for reducing the pressure Within the tank 10 within a very short time. The gas source 38 is so selected and constructed to provide dry, hot gas. The capacities ofthe refrigerator 26, the reclaimer 32, heat exchanger 33 and the pump 36 will become obvious from the detailed description that follows.

A shallow trough 37 is located at the bottom portion of the chamber 10. Trough 37 extends the full length of the chamber 10, is closed at both ends 12, 14 and is insulated from the chamber 10. A water-absorbing soluf, tion 44 is `placed in trough 37 and maintained at subfreezing water temperatures and at a level indicated by the dashed line 46.k In the specic embodiment of ythe invention shown in FIGS.' 1 to 3, this liquid 44 is in the form of saturated brine which is maintained at an approximate temperature between 5 C. to 40 C.

. This brine is recirculated through the chamber via the discharge pipe 30, the brine-reclaimer 32, the pump 36, the refrigerator 26, or heat exchanger 33,'and the line 28. The refrigerator 26 or heat exchanger 33 serves the purpose of maintaining the brine temperature. v

Two stacks 48 and 50 of green lumber are shown positioned inside the chamber 10 and as being supported on the two beams 20. Each of these stackss 48 and 50 is composed of a multiplicity of previously cut boards 52 which are spaced apart for drying in the usual manner. These boards may be cutV to any of the conventional dimensions. Itis customary to space the layers of boards 52 from each other by the use of Wood strips orsticks, this technique of spacing being commonly referred to in the art as sticking Suchwooden sticks which are spaced apart and inserted between boards 52in parallel relationship are indicated by `the' numeral 54.

It should be noted that the level 46 of the brine 44 vis below the level of the supporting beams 20 such that the lumber in the stacks 48 and 50 never come into direct contact with the brine. Also, as shown more clearlyin FIG. '1, the nozzle 42 is positioned in the end wall 12 such that dry gas at a superatmospheric temperature and pressure may be directed toward the stack 48ithrough and between the boards52 and onward to the stack 50. This particular arrangement of the nozzle 42 serves primarily in connection with explaining the -theory of this invention, a more practical arrangement being shown in FIGS. 5 yand 6 and described later on.

The apparatus illustrated and described thus far and the method which may be performed therewith thereby provides for the rapid drying of green liunber in either frozen or unfrozen states and overcomes the deficiencies of other methods of drying. In the present invention, use is made of evaporative freezing, and dehydration by sublimation `from the frozen state. This provides an extremely fast and economical drying means,` and in some instances lumber can be dried completely within twelve (12) hours or less. Y

Green lumber at ambient temperature and either in an unfrozen state or a frozen state, as represented by the two stacks 48 and 50, is first placed into the chamber 10.

, In the case of frozen lumber, the boards are first thawe lby long Wave radiation and immediately `thereafter the chamber is evacuated, as previously described, to start the drying process thereby preventing the initiationfof fungus growth which would result in staining of the boards.

volume of the board uniformly are to be used, radiant` heat from a steam coil surroundng the volume occupied by the boards being an example. Moisture in the boards 52 is drawn to the surfaces thereof by the pressure differ--L ential created during the evacuation of th-e chamber 10. Some of this moisture is vaporized. This evaporation is. attended by the removal of sufficient heat from the boards to produce ice crystals on the surfaces thereof.l The rate of freezing has a very definite bearing on the .size of the ice crystals so formed. Slow freezing produces large ice crystals and interspaces between crystals. The faster the freezing, the smaller the ice crystals and the interspaces. Even though the klarger interspacing of ice crystals appears to accelerate the subsequent drying, it is essential that the freezing take place as rapidly as possible to avoid the formation of large ice crystals which could break down the cellulose bers in the lumber. Thus, a large capacity, vacuum pumping system 22 is required to reach the operating pressure rapidly and then maintain that pressure while the drying process is continued. It is only necessary to reduce the pressure in the drying chamber 10 to the order of 10-1 millimeters of mercury at room temperature (20 C.) in order to create the environment suitable for the formation of ice crystals at the desired rate. Under these operating conditions, the capacity of the pump 22, therefore, must be suicient to maintain the pressure in the chamber 10 at least at the value of 10-1 milimeters of mercury.

The refrigerator 26 or heat exchanger 33 in conjunction with the circulating action of the pumps 36 and 22 should be sufficient to maintain the mass of the brine 44 in the chamber 10 at a temperature well below the freezing point of water, and for the exemplary operating condi-` tions above given, this temperature is to be held in the range from aboue 5 C. to 40 C.

As the relatively large mass of the brine 44 within the chamber 10 is thoroughly insulated from the shell of chamber 10, the internal temperature of the chamber will be maintained at an equilibrium temperature higher than the temperature of the brine.

As previously stated, the pumping of the chamber is suflciently rapid to assure the formation of small ice crystals. As the pressure inthe chamber is reduced, mois ture is drawn from Within each board to its surface, producing some evaporation,` with an attendant loss of heat. At a pressure in the region of 10-1 millimeters of mercury, the rate of evaporation produces sufficient heat loss to lower the temperature of the board surface to freezing (about 0 C.) and to for-m ice crystals on the board surfaces. It has been found that some 10% to 15% of the moisture content of the boards must be removed by evaporation for this freezing to occur at the above-meuf, tioned operating conditions. When ice begins to form, dry gas atta super-atmospheric pressure and temperature is admitted to the chamber via the nozzle 42, this nozzle 42 serving to direct the dry gas toward the ends of the two stacks 48 and 50, or in other Words through these stacks by passing between the boards from one end of the chamber yto theA other. In the specic operation described, the dryv gas may be super-heated steam at a pressure from about 1,0()150 p.s.i. gauge. The dry gas con` Y Alternatively, the frozen boards may be thawed outside of the drying chamber before placing therein and by means of the vacuum pump 22; however, most of the water vapor produced by the sublimation of the ice crystals cornes in contact with the cold brine because of the very large differential in water vapor pressure and condenses therebetween into the cold brine 44. Thus, as rapidly as small ice crystals form on the surfaces of the boards 52, they are sublimed by the dry steam. It is mandatory that these ice crystals not melt but instead pass immediately into the vapor state such that the boards 52 are never subjected to any wetting whereby they could warp or otherwise be damaged. As the volume of water vapor resulting from the sublimation process is very much greater than could be removed rapidly by any known vacuum system, it is very important to maintain a large temperature differential between the interior of the chamber and the brine 44. This temperature differential maintains a large vapor pressure differential thereby insuring the rapid flow of water vapor from the boards into the brine 44. The condensate absorbed by the brine 44 dilutes the brine therefore making necessary the removal of the condensate from the brine to permit maintenance of t-he low brine temperature without danger of ice formation.

This condensate removal is brought about by the use of `the reclaimer 32 which may take the form of a distilling apparatus, where fresh water is scarce; otherwise, it may consist of any natural method of evaporating water from brine. The diluted brine is fed through the reclaimer 32 on its way to the pump 36, the Water which is distilled or reclaimed therefrom being exhausted via the outlet 34. The concentrated brine is thereupon pumped from the reclaimer 32 onward to the refrigerator 26 or to the heat exchanger 33 (depending upon which embodiment of FIG. l is used) where the temperature is reduced to a value suitable to maintain the chamber 10 within the desired temperature range before being returned to the chamber it).

The process thus far described continues until all of the moisture is removed from the boards 52, the operation of the various components 22, 26 and 38 being so balanced as to provide the operating conditions by which the formation of relatively small ice crystals and the sublimation thereof occurs without permitting the boards themselves to freeze or reduce substantially in temperature.

In obtaining the above-mentioned proper balance in the operation of the various components, the temperature at the surfaces of the lumber should be maintained as closely as possible to a value of about 1 C. This is accomplished by controlling the pressure inside the chamber 10 by means of the pump 22 and also the volume of the subliming dry gas which is emitted by the nozzle 42. As above-mentioned, it is desirable to maintain the temperature of the brine within a value of about 5 C. to about 40 C. Under these operating conditions, a maximum vapor-pressure differential of approximately forty to one (40-to-1) between the lumber (4.26 mm. of mercury) and -the brine (0.1 mm. of mercury) is achieved. Accordingly, almost all of the aqueous vapor condenses into the solution 44, and the vacuum pump 22 is required to remove only the subliming gases and all non-water soluble vapors which may be Within the chamber 10. One reason for holding the temperature of the surfaces of the boards 52 near the above-mentioned -value of 1 C. is that vapor pressure changes rapidly with small changes in temperature in this region. The sublimation on the other hand must be maintained at a rate that will avoid permitting the boards to cool to the region of the solution temperature as this would result in the boards freezing, a vapor pressure decrease in the boards and a vastly retarded drying speed. A board temperature of about 1 C. is correct, for the above-mentioned operating conditions, to maintain the subliming and evaporation process at a desirable rate. However, this temperature as Well as the other operating conditions ofthe various components mayV vary without departing from the scope of this invention, so long as the above-mentioned results are achieved; these include the rapid formation of small ice crystals, the rapid sublimation thereof, the prevention of the boards from cooling to the point where they freeze and the removal of j the aqueous vapor resulting from the sublimation of ice crystals.

Instead of using beam supports 20 as shown in FIGS. l through 3, a more practical arrangement is the use of a narrow gauge track including the rails indicated by the numeral 56 in FIG. 4. These rails 56 are shown supported on suitable pedestals 59 secured to the bottom of the chamber 10 so that low, flat-bed trucks may be rolled into and out of the chamber 10 with ease through the doors 58.

Green lumber, as it comes from the banding or planing mills, is stacked on low, flat-bed trucks which travel on the narrow gauge track 56. Each layer of this green lumber is separated by sticks which assure free circulation of the dry gas to all boards and the withdrawal of the water vapor gas to the condensing solution and to the steam jet vacuum pump respectively. The use of the flat-bed trucks and the rails 56 obviously replace the beams 20 in the arrangement shown in FIGS. 1 and 2. The lumber may be stacked on the trucks in the same configuration as indicated by the numerals 48 and 50 and as previously described.

In further describing a working embodiment of this invention, sticks 54 as shown in FIGS. 5 and 6, instead of being made of wood and solid in form, are hollow, made of plastic, such as fiberglass reinforced epoxy resin, and rectangular tubes 60 having an inlet 63 on one side thereof and a series of nozzles or jet orifices 62 in the other side thereof. The orifices 62 are spaced apart along the length of the rectangular tube 60 and may be of a size of about 1/32 inch in diameter. There are a suicient number of these orifices 62 to assure a complete distribution of the dry gas to sublime all ice crystals as previously explained.

In the arrangement of FIGS. 5 and 6, the conduit 40 instead of being connected to a nozzle portion 42 is connected to the inlet nipple 63 of each of the tubular sticks 60. The sticks 60 are so directed along the surfaces of the boards so as to sublime all of the ice crystals from the surfaces thereof. By the use of these sticks 60, adequate penetration of the dry gas throughout the stacks 4S and 50 and heating of the board surfaces is assured. In FIG. 6, a jet 64 of gas is shown issuing from one of the orifices 62.

In the lumbering industry, where it is necessary to dispose of waste products such as slab wood, sawdust and shavings, any green lumber drying system which could utilize these Waste materials to advantage would provide a major economic gain. A steam jet, vacuum pump systern with a dry, low-pressure, steam convective dehydrating system, therefore, is one of the most desirable systems for pumping the necessary vacuum and is the one used in the working embodiment of this invention. In those situations where water is scarce or expensive, the large volumes which are removed from the green lumber are recovered from the vapor-condensing solution 44 for use as boiler feed water.

The steam-jet, vacuum pump, well known in the art, operates on the principle that steam expanding through a nozzle has its pressure energy `converted to velocity energy thereby developing a relatively low pressure in the rapidly moving expanding steam. The high velocity steam jet thereby sucks gases and vapors into its high velocity stream. The resulting mixture enters the converging end of a diffuser Where it is compressed into a pressure less than atmospheric. The combination of unit cost and stage efficiency characteristics determines the economically feasible stage compression ratios. Generally, the arrangements using more stages have higher first costs and lower steam and water consumption. For maintaining the above-mentioned operating conditions, a ve stage steam jet vacuum system with a maximum pumping speed of 10,000 liters per second at 10-1 torr has been found to be satisfactory. This system employs a primary-booster ejector, a secondary-booster ejector, a tertiary-booster ejector, a high-vacuum ejector, a low-vacuum ejector, and

three ordinary condensers, one between the high-vacuum and low-vacuum ejectors, and one following the lowvacuum ejector (after the condenser). The primary and secondary booster-ejectors are steam jacketed to prevent buildup of ice on the diffuser internal bore which would interfere` with the proper operation of the boosters. The condensers are cooled by the circulating solution from the vacuum drying chamber 10, and the condensate is returned to the water-supply storage. The steam-jet, vacuum pump system may be connected to a drying chamber by suitable vacuum valves.

The reclaiming apparatus 32 may take the form of shallow evaporative storage ponds which are open to the atmosphere. Such ponds are covered to prevent dilution of the brine by rain water. Excess water is evaporated from the diluted brine as the temperature rises to ambient. Once the excess water or condensate is evaporated from the brine, the latter is pumped back to the drying cham- Vber 10 via the refrigerator 26 which cools the same to the proper temperature.

As aforementioned, a heat exchanger 33, in a modification of this invention, can be substituted for the refrigerator 26 as shown in the dotted lines of FIG. 1. In this modification, heat exchanger33 is inserted in discharge line 30 and `conduit 28, so that the liquid from chamber 10 passes through one side of heat exchanger 33 on its way to reclaimer 32 and so that the liquid upon leaving reclaimer 32 passes through the other side of heat exchanger 33 on its way back to -chamber 10. Heat exchanger 33 is conventional in all respects and may be any exchanger having two opposite sides to maintain liquid segregation and having the required heat exchange capacity. The liquid, in this arrangement, upon leaving chamber 10 is warmed during its passage through heat exchanger 33 before passing on to reclaimer 32. The liquid leaving reclaimer 32 is similarly cooled during its passage through heat `exchanger 33 on its way back to chamber 10. The heat measured in B.t.u., or similar units, gained by the liquid leaving chamber 10 is substantially equal to the heat lost by the liquid entering chamber 10. Heat losses in this arrangement are made up by continued evacuation of chamber 10 by vacuum pump 22, thereby to maintain the liquid in chamber 10 in the desired temperature range.

A number of drying solutions 44 have been Ifound to be 'eiective in supplementing the vaecum system for removal of the aqueous vapor `from the chamber and in maintaining the sublimation process. These 'include any kind of soluble salts, ethyl alcohol, ethyleneV glycol, or any non-inflammable solvents with a high solubility for water. If the lumber-drying mill is located on brackish or sea-water, the salinity may be such that it will be adequate or made so by the addition of anti-freeze materials.

The apparatus of this invention can alsobe used to impregnate lumber with desirable dyes, Sealers, finishes, fungicides, moisture inhibitors and the like. In this modiication of this invention, suitable impregnant tanks are situated externally of the chamber 10 but have suitable communication therewith by means of connecting conduits. These tanks have steam-jackets `for the purpose of heating the contents thereof. By heating the contents of these tanks, the vapor pressures of the dyes and impregnants as well as the rates of evaporation thereof mayk be increased. In operation, these impregnant tanks are connected to the drying chamber through appropriate piping in-valves to the steam manifolds or sticks 60. By .valving olf the steam 38, 40 and opening the dye or be admitted to the vdrying chamber at greater thanatmospheric pressures. The vapors expand upon Vexit through the nozzle orifices 62 into the vacuum chamber 10 and penetrate the voids in the boards 52 created `by the removal of water. The depth of penetration into the boards may be controlled by the duration of the exposure of the lumber to the die or impregnant and by the vapor pres-` sure of the impregnant in chamber 10. In order to con-1 duced by the removal of moisture and then condense,- upon coming into contact with the cold surfaces of the` boards, thereby providing the lumber with properties which were not present in the original raw material. The operator may control the amount of impregnation by the duration ofthe exposure of the lumber to the impregnant and by the amount of impregnant released in the chamf. ber 10. In this manner, the boards may be lightly treated,

sealed against moisture absorption, or completely plasticized. For example, the pre-finishing of hardwood ooring can be accomplished in the vacuum-drying chamberv 10. In connection with this, the boards are first colored, and then impregnated with a plasticizer such as urethane to seal out Vmoisture and to provide a hard scutf-proof and lasting surface nish.

From the foregoing, it will now be apparent that drying of green lumber to a uniform moisture content as low as 2% in Afrom four to twelve hours is possible. The drying process does not alter the characteristics of the wood. The wood bers are not weakened or distorted. Tight knots are not loosened. Wood color is not altered, and the grainr is not raised. Obviously, these characteristics will vary with different species of woods, but, in the main, improvements in these respects in drying these according to the teaching of this invention are realized.

The lumber may be impregnated to any degree desired during the drying process in following the principles of the present invention. The equipment used in thisinvention is reliable and easily maintained. Woods may be finished to customer specifications on short notice and inventories can be reduced substantially thereby eliminat-` ing the need for large warehouses and storage areas. This provides a reduction in insurance costs and labor-handling charges. Therefore substantial economiesare realize in the practice of this invention.

While there have been described above the principles of this invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention.

What is claimed is:

1. Apparatus for drying lumberV comprising a sealed chamber, water-vapor absorber in said chamber, a support mounted in said chamber for supporting and bearing the weight of a stack of lumber to be dried, means connected to said chamber for reducing the gas pressure therein to a predetermined value, means for cooling said absorber and for maintaining the same at a tem-- perature below the temperature of` lumber placed in said Y chamber, means for removing water from said absorberY so as to maintain the concentration thereof relatively;

constant, said pressure-reducing` means having a capacity land said absorber being maintained at a temperature that moisturein green lumber is vaporized at a rate sul'icient to lower vthe temperature at the surface of the lumber Yto a point at which ice crystals are formed, means for heating the lumber surface and subliming the ice crystals, said heating means being related to said pressure-reducing means and the temperature of said absorber that said lumber is never Ycooled throughout the mass thereof to the temperature of said absorber and that only the lumber surface is cooled enough to form ice crystals.

2. The apparatus of claim 1 further comprising a trough within said chamber, said trough being adjacent to and insulated from said chamber, and wherein said water-vapor absorber is salt brine located within said trough, said pressure-reducing means is a vacuum pump,

said heating means includes a steam generator capable Vof producing dry superheated steam, and means for injecting said steam into said chamber whereby the surfaces of lumber may be heated.

3. The apparatus of claim 1 further comprising a trough within said chamber, said trough being .adjacent to and insulated from said chamber, and wherein said watervapor absorber is salt brine located within said trough, said pressure-reducing means is a vacuum pump, said heating means comprises a heater for generating dry gas, and means for injecting dry gas into said chamber whereby the surfaces of the lumber may be heated.

4. The apparatus of claim 1 further comprising a trough within said chamber, said trough being adjacent to and insulated from said chamber, and wherein said water-vapor absorber is salt brine located within said trough, said pressure-reducing means is a vacuum pump, said heating means comprises a dry gas generator, means for injecting dry gas into said chamber, and means for heating said chamber and for radiantly heating said dry gas whereby ythe surfaces of the lumber may be heated.

5. Apparatus for drying lumber comprising a sealed chamber having doors 4thereon providing access to the interior of said chamber, said chamber having car-Supa porting means therein whereby lumber stacked on a car may be positioned inside said chamber, a water-Vapor absorber in the bottom portion of said chamber disposed below said car-supporting means, said water-vapor absorber being insulated from said chamber, a multi-stage steam-jet vacuum pump connected to said chamber for reducing the pressure therein to a predetermined value, a refrigerating means in contact with said absorber for maintaining the same at a temperature below the freezing point of water, means for removing water from said absorber at a predetermined rate, said vacuum pump having a capacity and said absorber being in suiiicient quantity and being maintained at a temperature that moisture in green lumber is vaporized from and frozen into ice crystals on the lumber surface, a source of dry superheated steam communicating with the interior of said chamber and including means for distributing the steam throughout the lumber inside said chamber, said source delivering steam 4at a temperature and in an amount at which ice crystals on the surface of said lumber are sublimed without heating the surface of said lumber to a temperature above the freezing point of water, whereby moisture may be continuously removed from said lumber until the later is dried.

6. The apparatus of claim 5 wherein said vacuum pump has a capacity for maintaining the chamber pressure at about 1 to about 10-2 millimeters of mercury, and said refrigerating device has a capacity of maintaining 4the temperature of said solution in the range of from about 10 C. 4to about 40 C.

7. The method `of drying green lumber comprising the steps of placing a quantity of green lumber into a sealed chamber, reducing the pressure of said chamber to a predetermined value which is less than the Vapor pressure of the moisture in said lumber and reducing the temperature within said chamber to a predetermined value below the freezing point of water, said pressure being reduced at a rate Iat which moisture in said lumber is vvaporized and ice crystals are formed on the lumber surface; subliming said ice crystals and simultaneously maintaining the internal temperature of said lumber above the freezing point of water, and removing from said chamber the water vapor which results from subliming the ice crystals.

-10 8. The method of drying green lumber comprising the steps of placing a quantity of green lumber into 4a sealed chamber, reducing the pressure of said chamber to a predetermined value which is less than the vapor pressure of the moisture in said lumber and simultaneously reducing the temperature within said chamber to a predetermined value below the freezing point of water, said pressure being reduced at a rate at which moisture in said lumber is vaporized and ice crystals are formed on the lumber surface, ,and subliming said ice crystals and maintaining the internal temperature of said lumber such that the latter is not frozen.

9. The method of drying green lumber comprising the steps of placing a quantity of green lumber into a sealed chamber, reducing the pressure of said chamber to a predetermined value which is less than the vapor pressure of the moisture in said lumber and reducing the temperature within said chamber to a predetermined value below the freezing point of water, said pressure being reduced at a rate at which moisture in said lumber is vaporized and ice crystals are formed on the lumber surface; directing dry gas over the surface of said lumber in such quantity at such a rate that the ice crystals are sublimed without melting and the interior of the lumber is maintained .at a temperature at which the moisture inside the lumber does not freeze and removing the sublimation products from ythe chamber.

10. The method of drying green lumber comprising the steps of placing a quantity of green lumber into a sealed chamber, introducing into said chamber a controlled volume of water-vapor absorbing liquid, cooling said liquid to a predetermined temperature below the freezing point of water, reducing the pressure of said chamber to a predetermined value which is less than the vapor pressure of the moisture in said lumber and at which ice crystals form on the lumber surface from moisture contained by said lumber yand greater than the vapor pressure of the absorber, subliming said ice crystals and maintaining the internal temperature of said lumber above the freezing point of water, said volume of liquid absorber being sufficient to absorb the condensate from the sublimed ice, withdrawing said liquid and condensate from said chamber, removing the .condensate from said liquid following withdrawal thereof from said chamber and thereafter returning the liquid to said chamber.

11. The method of drying green lumber comprising the steps of placing a quantity of green lumber into a sealed chamber, introducing into said chamber a controlled volume of water-vapor absorbing liquid, cooling said liquid to a predetermined temperature below the freezing point of water, reducing the pressure of said chamber to a predetermined value which is less than the vapor pressure of the moisture in said lumber and at which ice crystals form on the lumber surface from moisture contained by said lumber, directing dry superheated steam over the surface of said lumber in su-ch quantity at such a rate thatpthe ice crystals are sublimed without melting and .the interior lof the lumber is maintained at a temperature at which the moisture inside said lumber does not freeze, and removing the sublimation products from the chamber.

12. The method of claim 7 and including the introduction of vaporized impregnating material Iinto said chamber 4after the lumber has been dried, whereby said impregnating material will be absorbed by said lumber.

13. The method of claim 10 and including the step of introducing an impregnating vapor into said chamber at a pressure greater than the aforesaid reduced chamber pressure, whereby the differential pressure internally of the dried lumber and the impregnating vapor permits the impregnating vapor to penetrate the same.

14. The method of claim 9 wherein said lumber comprises a number `of lumber pieces, spacing said pieces 011ey from the other, and directing said dry gas between said pieces to sublime `the ice on the surface thereof.

1 1 1 2 15. The method of claim 10 wherein the average sur- 1,763,070 6/ 1930 Shinn 34-16.5 face temperature of the lumber is maintained at about 2,296,546 9/1942 Toney 34-16.5 l 0 C. and the average temperature of the liquid 4is main- 2,391,441 12/1945 Baer 34-15 Itained below 5 C. 2,435,218 2/1948 Hudson 34-37 5 435 5 References Cited by the Examiner 5284g slsol UNITED STATES PATENTS 2,802,281 8/1957 Stone 34-92 1,219,406 3/1917 Banks 34-16.5 2,860,070 11/1958l McDonald 34-95 1,497,362 6/ 1924 Dhe 34-37 1,672,326 6/1928 Kobiolke 34 16.5 10 WILLIAM J. WYE,Prima1y Examiner. 

1. APPARATUS FOR DRYING LUMBER COMPRISING A SEALED CHAMBER, WATER-VAPOR ABSORBER IN SAID CHAMBER, A SUPPORT MOUNTED IN SAID CHAMBER FOR SUPPORTING AND BEARING THE WEIGHT OF A STACK OF LUMBER TO BE DRIED, MEANS CONNECTED TO SAID CHAMBER FOR REDUCING THE GAS PRESSURE THEREIN TO A PREDETERMINED VALUE, MEANS FOR COOLING SAID ABSORBER AND FOR MAINTAINING THE SAME AT A TEMPERATURE BELOW THE TEMPERATURE OF LUMBER PLACED IN SAID CHAMBER, MEANS FOR REMOVING WATER FROM SAID ABSORBER SO AS TO MAINTAIN THE CONCENTRATION THEREOF RELATIVELY CONSTANT, SAID PRESSURE-REDUCING MEANS HAVING A CAPACITY AND SAID ABSORBER BEING MAINTAINED AT A TEMPERATURE THAT MOISTURE IN GREEN LUMBER IS VAPORIZED AT A RATE SUFFICIENT TO LOWER THE TEMPERATURE AT THE SURFACE OF THE LUMBER TO A POINT AT WHICH ICE CRYSTALS ARE FORMED, MEANS FOR HEATING THE LUMBER SURFACE AND SUBLIMING THE ICE CRYSTALS, SAID HEATING MEANS BEING RELATED TO SAID PRESSURE-REDUCING MEANS AND THE TEMPERATURE OF SAID ABSORBER THAT SAID LUMBER IS NEVER COOLED THROUGHOUT THE MASS THEREOF TO THE TEMPERATURE OF SAID ABSORBER AND THAT ONLY THE LUMBER SURFACE IS COOLED ENOUGH TO FORM ICE CRYSTALS. 